The so-called Swiss-type automatic lathe is well known as a compact, highly accurate lathe for machining a workpiece which is long with respect to its diameter. This lathe possesses a characteristic structure comprising a sliding headstock, a guide bushing disposed in the direction of movement of the headstock, and five or six tool holders for the turning operation disposed about the guide bushing as a center and radiating outwardly therefrom. A Swiss-type automatic lathe utilizing a cam system in which the movement of the headstock and tool holders is controlled by cams is being widely used as a specialized machine for mass production. In recent years, however, the adoption of a numerical control system has made it possible to employ this lathe in medium and small-scale production as well. A number of numerically controlled lathes employing the cam system can be mentioned. In one system the rotation of the cam shafts is numerically controlled. In another, the radially disposed tool holders are individually driven by numerically controlled lead screws. Still another system makes use of the tool holders radially disposed about the guide bushing and employs a single numerically controlled cam which rotates about the guide bushing.
These systems all have a common drawback. Namely, since the tool holders are radially disposed and must be individually and sequentially moved, the individual supporting guideways must be small in size; hence, the tool holders do not possess sufficient rigidity.
The adoption of a numerical control system which makes possible a wide variety of production configurations leads to an increase in the kind of workpieces which can be machined. As a result, there is now a necessity for the machining of workpieces which have an enlarged diameter in comparison to such miniature components as timepieces which have mainly been machined by the Swiss-type automatic lathe. The maximum machining diameter which determines the capacity of the lathe has been increased to the point where the characteristic fundamental structure of the lathe cannot be maintained because of insufficient rigidity. There is also a demand for high productivity and hence a requirement that actual machining time and idle time be reduced. Insufficient rigidity, however, makes reduction of actual machining time a difficult goal to attain. Research has shown that, in a case where total working time exceeds two minutes, actual machining time occupies 80% of total working time and cannot be reduced because of the insufficient rigidity of the Swiss-type automatic lathe.
In an effort to overcome these shortcomings a numerically controlled automatic lathe of sliding headstock type, actually an improved Swiss-type automatic lathe, has already been proposed. The lathe is provided with sufficient rigidity by replacing the radially disposed tool holders with a turret head system and consolidating the guideways of the tool holders. This makes it possible to increase the maximum machining diameter up to approximately 30 mm which is the upper limit of the actual machining diameter that can be machined by a prior art machining method characteristic of machines that employ a guide bushing. Moreover, sufficient rigidity permits powerful machining so that actual machining time and the proportion of the total working time which it occupies can be greatly reduced.
However, with still greater demand for higher productivity, a problem arises in reducing the increased idle time ratio resulting from the reduction in the actual working time radio. Specifically, the conventional numerically controlled automatic lathe of sliding headstock type employing the turret head system possessed a serious drawback in that the demand for higher productivity could not be met since a reduction in idle time was limited by the great amount of time needed to index and position the turret. In order to shorten the indexing time it is important not only to raise the rotating speed of the turret head but also to reduce the acceleration and deceleration time needed to start and stop its rotation. It is therefore desirable to reduce the inertia of the turret head to the greatest possible extent, in other words, to minimize the rotating radius of the turret head.
This causes problems, particularly in a lathe of the type having a guide bushing, in which the turret head when arranged such that the path of each tool cutting edge is positioned at the mouth of the guide bushing, has a large rotating radius.
On the other hand, because of the increasing requirement for lathes, especially numerically controlled lathes, that can perform a wide variety of operations, it is desirable to mount as many tools on the turret head as possible, a measure that increases the number of turret head mounting surfaces to which tool supporting fixtures are attached. If it is assumed that the area of each mounting surface cannot be reduced below a certain level because of a loss of rigidity, the size of the turret shaft would inevitably be enlarged by increasing the number of tools, resulting in an increase of the turning radius of the head.